Cathode active material for lithium secondary battery

ABSTRACT

Provided is a lithium transition metal oxide having an α-NaFeO 2  layered crystal structure, as a cathode active material for lithium secondary battery, wherein the transition metal includes a blend of Ni and Mn, an average oxidation number of the transition metals except lithium is +3 or higher, and the lithium transition metal oxide satisfies Equations 1 and 2 below:
 
1.0&lt; m (Ni)/ m (Mn)  (1)
 
 m (Ni 2+ )/ m (Mn 4+ )&lt;1  (2)
         wherein m(Ni)/m(Mn) represents a molar ratio of nickel to manganese and m (Ni 2+ )/m(Mn 4+ ) represents a molar ratio of Ni 2+  to Mn 4+ .       

     The cathode active material of the present invention has a uniform and stable layered structure through control of oxidation number of transition metals to a level higher than +3, in contrast to conventional cathode active materials, thus advantageously exerting improved overall electrochemical properties including electric capacity, in particular, superior high-rate charge/discharge characteristics.

This application is a continuation of PCT/KR2010/003883 filed on Jun.16, 2010, which claims priority to Korean Application No.10-2009-0054106 filed on Jun. 17, 2009. The entire contents of the aboveapplications are hereby incorporated by reference into the presentapplication.

TECHNICAL FIELD

The present invention relates to a cathode active material for lithiumsecondary batteries. More specifically, the present invention relates toa cathode active material which is a lithium transition metal oxidehaving an α-NaFeO₂ layered crystal structure, wherein the transitionmetal includes a blend of Ni and Mn, an average oxidation number of thetransition metals except lithium is higher than +3, and a molar ratio ofnickel to manganese (m(Ni)/m(Mn)) and a molar ratio of Ni²⁺ to Mn⁴⁺(m(Ni²⁺)/m(Mn⁴⁺)) satisfy specific conditions. The lithium transitionmetal oxide has a uniform and stable layered structure through controlof the oxidation number of transition metals, thus exerting superiorrate characteristics under high-rate charge/discharge conditions as wellas improved overall electrochemical properties.

BACKGROUND ART

Technological development and increased demand for mobile equipment haveled to a rapid increase in the demand for secondary batteries as energysources. Among these secondary batteries, lithium secondary batterieshaving high energy density and voltage, long cycle span and lowself-discharge are commercially available and widely used.

In addition, increased interest in environmental issues has broughtabout a great deal of research associated with electric vehicles, hybridelectric vehicles and plug-in hybrid electric vehicles as substitutesfor vehicles using fossil fuels such as gasoline vehicles and dieselvehicles. These electric vehicles generally use nickel-metal hydridesecondary batteries as power sources. However, a great deal of studyassociated with use of lithium secondary batteries with high energydensity and discharge voltage is currently underway and some arecommercially available.

Meanwhile, the lithium secondary batteries generally uselithium-containing cobalt composite oxide (LiCoO₂) as a cathode activematerial. Also, the use of lithium-manganese composite oxides such asLiMnO₂ having a layered crystal structure and LiMn₂O₄ having a spinelcrystal structure and lithium nickel composite oxide (LiNiO₂) as thecathode active material has been considered.

Among these cathode active materials, LiCoO₂ is the most generally usedowing to superior physical properties such as long lifespan and goodcharge/discharge characteristics, but has low structural stability andis costly due to natural resource limitations of cobalt used as a rawmaterial, thus disadvantageously having limited price competiveness.

Lithium manganese oxides such as LiMnO₂ and LiMn₂O₄ have advantages ofsuperior thermal stability and low costs, but have disadvantages of lowcapacity and bad low-temperature characteristics.

In addition, LiMnO₂-based cathode active materials are relatively cheapand exhibit battery characteristics of superior discharge capacity, butare disadvantageously difficult to synthesize and are unstable.

In order to solve the afore-mentioned problems, the present inventionprovides a low-cost highly functional cathode active material comprisinglithium transition metal composite oxide wherein constituent elementssatisfy requirements including a predetermined composition and oxidationnumber, as mentioned below.

In this regard, U.S. Pat. No. 6,964,828 discloses a lithium transitionmetal oxide having a structure of Li(M1_((1−x))-Mn_(x))O₂ wherein M1 isa metal other than Cr, and each Ni has an oxidation number of +2, eachCo has an oxidation number of +3, and each Mn has an oxidation number of+4, provided that M1 is Ni or Co.

In addition, Korean Patent Laid-open No. 2005-0047291 suggests a lithiumtransition metal oxide wherein Ni and Mn are present in equivalentsamounts and have an oxidation number of +2 and +4, respectively.

As another example, Korean Patent No. 543,720 discloses a lithiumtransition metal oxide wherein Ni and Mn are present in substantiallyequivalent amounts, Ni has an oxidation number of 2.0 to 2.5 and Mn hasan oxidation number of 3.5 to 4.0. This patent discloses that Ni and Mnshould substantially have an oxidation number of +2 and +4,respectively, and that lithium batteries are deteriorated in properties,unless Ni and Mn have an oxidation number of +2 and +4, respectively, asapparent from Examples and Comparative Examples.

Also, Japanese Patent Application Publication No. 2001-0083610 disclosesa lithium transition metal oxide which is represented by a structure ofLi((Li(Ni_(1/2)Mn_(1/2))_((1−x)))O₂ orLi((Li_(x)(Ni_(y)Mn_(y)Co_(P))_((1−x)))O₂ and contains Ni and Mn inequivalent amounts. In accordance with the application, provided that Niand Mn are present in identical amounts, Ni and Mn form Ni²⁺ and Mn⁴⁺,respectively, realizing structural stability and thus providing thedesired layered structure.

Accordingly, in accordance with the related art as mentioned above, theaverage oxidation number of transition metals should be +3, which ismentioned in U.S. Pat. No. 7,314,682. In this patent, the inventorsdisclose lithium transition metal oxide represented by the structure ofLi_((2+2x)/(2+x))M′_(2x(2+x)/(2+x))M_((2−2x)/(2+x))O_(2−δ) wherein M′ isan element having an average oxidation number of +3, in which M′ is nota Li metal, and M is a transition metal having an average oxidationnumber of +3.

As can be confirmed from the afore-mentioned related patents, it wasconventionally believed that (i) transition metals should have anaverage oxidation number of +3 in order to impart a stable layeredstructure to lithium transition metal oxide, and (ii) Ni present in anamount equivalent to Mn⁴⁺ should have an oxidation number of +2 in orderto impart superior electrochemical properties to the lithium transitionmetal oxide, based on premise (i).

However, the inventors of the present application confirmed that, in thecase where Mn⁴⁺ and Ni²⁺ are simply selected to obtain an averageoxidation number of +3, Ni²⁺ or the like is transferred to reversible Lisites, the problem, deterioration in electrochemical properties, cannotbe solved.

Meanwhile, U.S. Pat. Nos. 7,078,128 and 7,135,252 suggest materialswherein Mn is present in an amount higher than that of Ni. However, theinventors of the present invention confirmed based on test results thata high amount of Mn cannot change an oxidation number of Mn⁴⁺ uponLi-charging, thus causing a decrease in capacity.

Meanwhile, it is generally known that the case, in which Co is present,maintains superior structural stability than the case in which Co is notpresent. However, as mentioned above, Co is more expensive than Ni, Mnor the like and attempts continue to be made to reduce use thereof.Unless the afore-mentioned specific conditions are satisfied, superiorperformance cannot be exerted, and although active materials satisfyingthe requirements are actually synthesized, they exhibit poorelectrochemical properties such as decrease in capacity anddeterioration in rate properties.

DISCLOSURE Technical Problem

Therefore, the present invention has been made to solve the aboveproblems and other technical problems that have yet to be resolved andit is one aspect of the present invention to provide a cathode activematerial with superior structural and electrochemical properties.

As a result of a variety of extensive and intensive studies andexperiments to solve the problems as described above, the inventors ofthe present invention have discovered that in the case where a cathodeactive material is based on a lithium transition metal oxide having alayered crystal structure wherein the transition metal has an averageoxidation number higher than +3, the content of nickel is higher thanthat of manganese, and the content of Ni²⁺ is lower than that of Mn⁴⁺,and the cathode active material has a complete crystal structure, thusconsiderably improving high-rate charge/discharge characteristics. Thepresent invention was completed, based on this discovery.

Technical Solution

Accordingly, the cathode active material for lithium secondary batteriesaccording to the present invention is a lithium transition metal oxidewhich has an α-NaFeO₂ layered crystal structure, wherein the transitionmetal includes a blend of Ni and Mn, an average oxidation number of alltransition metals except lithium is +3 or higher, and the lithiumtransition metal oxide satisfies Equations 1 and 2 below:1.0<m(Ni)/m(Mn)  (1)m(Ni²⁺)/m(Mn⁴⁺)<1  (2)

-   -   wherein m(Ni)/m(Mn) represents a molar ratio of nickel to        manganese and m (Ni²⁺)/m(Mn⁴⁺) represents a molar ratio of Ni²⁺        to Mn⁴⁺.

As mentioned above, it was conventionally known in the art that anaverage oxidation number of transition metal ions should be adjusted to+3 by adding Ni²⁺ and Mn⁴⁺ in equivalent amounts in order to obtain adesired layered structure. However, since Ni²⁺ has a size substantiallysimilar to Li⁺, it moves to the lithium layer and readily forms mineralsalts, thus disadvantageously causing deterioration in electrochemicalproperties.

Accordingly, the inventors of the present invention conducted a greatdeal of research to prepare a cathode active material which has a stablelayered crystal structure and exhibits superior capacity and ratecharacteristics. As a result, the inventors discovered that thestability of the layered crystal structure depends on the sizedifference between the lithium ion and the transition metal ion, ratherthan Ni²⁺ and Mn⁴⁺.

Specifically, the inventors confirmed that lithium composite transitionmetal oxide having a layered crystal structure of α-NaFeO₂ is dividedinto a lithium-containing Li-oxide layer (referred to as a “lithiumlayer”) and a transition metal-containing transition metal oxide layer(referred to as an “MO layer”). As the size difference between the ionsconstituting respective layers, that is, the size difference between thelithium and transition metal ions, increases, the more easily can thetwo layers be separated and developed.

The inventors of the present invention continually tried to accomplishthe desired layered crystal structure. As a result, the inventorsconfirmed that the size difference between the ions may be indicated bythe bonding distance between each ion and the oxygen ion or bondingforce therebetween, and as the oxidation number of a metal havingcationic characteristics increases, ionic diameter decreases.Accordingly, the inventors considered that the difference between the MOlayer and the lithium layer can be increased by increasing the oxidationnumber of transition metals. This expectation was confirmed through agreat deal of experiments.

The principle that the layered crystal structure can be suitablydeveloped through increased size difference between the lithium ion andthe transition metal ion by increasing the average oxidation number ofthe transition metal to a level higher than +3 is in contrast to theconventional idea accepted in the art that the average oxidation numberof transition metals should be adjusted to +3 to stabilize the layeredcrystal structure.

Meanwhile, the case where the contents of Ni and Mn are substantiallyequivalent in a conventional manner is undesirable in that Mn⁴⁺ inducesformation of Ni²⁺ and, in compounds containing a great amount of Mn, agreat amount of Ni²⁺ is arranged in the lithium layer.

Accordingly, the inventors of the present invention predicted that thebest method to increase the oxidation number of transition metals wouldbe to adjust the total average oxidation number to +3 or higher bydecreasing the amount of Ni²⁺, which can be readily permeated into thelithium layer. It was considered that the amount of Ni³⁺ having a sizesmaller than Ni²⁺ increases, thus causing an increase in size differencebetween the ions.

Accordingly, the cathode active material according to the presentinvention, as mentioned above, contains nickel and manganese whereinnickel is present in an amount higher than manganese (see Equation (1))and Ni²⁺ is present in an amount smaller than Mn⁴⁺ (see Equation (2)).

Accordingly, the cathode active material of the present invention is alithium nickel manganese oxide wherein (i) an average oxidation numberof nickel and manganese except lithium is greater than +3, (ii) whereinmore nickel is present than manganese and (iii) less Ni²⁺ is presentthan Mn⁴⁺.

Advantageously, such lithium manganese oxide maintains the averageoxidation number of transition metals to a level higher than +3, thusconsiderably decreasing the amount of transition metals present in thelithium layer, based on the stable crystal structure of the cathodeactive material, thereby improving mobility and rate characteristics oflithium ions, as well as capacity.

Regarding aspect (i), the cathode active material of the presentinvention has an average oxidation number of transition metals exceptlithium, higher than +3, thus decreasing an average size of transitionmetal ions, increasing the size difference between lithium ions, andpromoting separation between layers, thereby forming a stable layeredcrystal structure.

Preferably, the average oxidation number of total transition metalsexcept Li is higher than 3.0 and not higher than 3.5, more preferably,3.01 to 3.3, more particularly preferably, 3.1 to 3.3.

Regarding aspect (ii), the cathode active material according to thepresent invention is composed of nickel and manganese wherein thecontent of nickel is higher than that of manganese, on a molar basis, asrepresented by Equation 1 below:1.0<m(Ni)/m(Mn)  (1)

In the case where nickel is present in an amount higher than manganese,nickel in an amount corresponding to the difference between the nickelcontent and the manganese content, is changed to Ni³⁺, which has arelatively small ionic size.

Accordingly, the average size difference between the lithium ion and thetransition metal ion increases, thus minimizing intercalation of Ni²⁺into the lithium layer and improving stability of the layered crystalstructure.

On the other hand, when manganese is present in an amount higher thannickel, +4 ions which do not enhance charge/discharge characteristicsare increased and capacity is thus decreased.

As mentioned above, in the case where the cathode active materialaccording to the present invention contains excess nickel, as comparedto manganese, the nickel is divided into nickel (a) present in anexcessive amount, as compared to the manganese content and nickel (b)present in an amount corresponding to the manganese content.

Preferably, the nickel (a) present in an excessive amount, as comparedto the manganese content is Ni³⁺, and the nickel (b) present in anamount corresponding to the manganese content contains Ni²⁺ and Ni³⁺.

Regarding aspect (iii), the cathode active material according to thepresent invention is composed of nickel and manganese wherein a molarratio of Ni²⁺ to Mn⁴⁺ (m(Ni²⁺)/m(Mn⁴⁺)) is lower than 1 (that is, Ni²⁺and Mn⁴⁺ are not present in equivalent amounts and Ni²⁺ is present in anamount smaller than Mn⁴⁺), as represented by Equation 2 below:m(Ni²⁺)/m(Mn⁴⁺)<1  (2)

When the molar ratio of Ni²⁺ to Mn⁴⁺ is lower than 1 (that is, thecontent of Ni²⁺ is equivalent to or higher than that of Mn⁴⁺), theaverage oxidation number of transition metals does not increase andcannot induce the difference in ion size. In the case wherem(Ni²⁺)/m(Mn⁴⁺) is higher than 0.4 and lower than 0.9, considerablysuperior electrochemical properties can be obtained.

As such, for the cathode active material according to the presentinvention, the nickel content is equivalent to or higher than themanganese content and an average oxidation number of transition metalsis higher than +3, thus increasing the size difference between thelithium ion and the transition metal ion, promoting layer separation andminimizing permeation of Ni²⁺ into the lithium layer. For the cathodeactive material, the content of nickel intercalated into the lithiumsite may be lower than 5 mol %.

For the lithium transition metal oxide of the present invention,transition metals comprising nickel, manganese and optionally cobalt canbe partially substituted with other metal element (s) within an amount,so long as the layered crystal structure can be maintained, preferablyan amount not higher than 20% with a metal element (including transitionmetal) or a cationic element, more preferably an amount not higher than10% with a metal element (including transition metal) or a cationicelement, based on the mole of the transition metal. It is apparent thatthis case is included within the scope of the present invention so longas the properties of the present invention are satisfied.

The present invention provides a positive electrode comprising thecathode active material and a lithium secondary battery comprising thepositive electrode. Hereinafter, the positive electrode is simplyreferred to as a “cathode”.

The lithium secondary battery generally comprises a cathode, an anode, aseparator, and a lithium salt-containing non-aqueous electrolyte.

For example, the cathode is prepared by applying a cathode mixcomprising a cathode active material, a conductive material, a binderand a filler to a cathode current collector, followed by drying. Thecathode mix may comprise a filler, if necessary.

The cathode current collector is generally fabricated to have athickness of 3 to 500 μm. There is no particular limit to the cathodecurrent collector, so long as it has suitable conductivity withoutcausing adverse chemical changes in the fabricated battery. As examplesof the cathode current collector, mention may be made of stainlesssteel, aluminum, nickel, titanium, sintered carbon, and aluminum orstainless steel surface-treated with carbon, nickel, titanium, silver orthe like. If necessary, these current collectors may also be processedto form fine irregularities on the surface thereof so as to enhanceadhesion to the cathode active materials. In addition, the currentcollectors may be used in various forms including films, sheets, foils,nets, porous structures, foams and non-woven fabrics.

The conductive material is commonly added in an amount of 1 to 40% byweight, based on the total weight of the mixture including the cathodeactive material. Any conductive material may be used without particularlimitation so long as it has suitable conductivity without causingadverse chemical changes in the fabricated secondary battery. Asexamples of the conductive materials that can be used in the presentinvention, mention may be made of conductive materials, includinggraphite such as natural or artificial graphite; carbon blacks such ascarbon black, acetylene black, Ketjen black, channel black, furnaceblack, lamp black and thermal black; conductive fibers such as carbonfibers and metallic fibers; metallic powders such as carbon fluoridepowder, aluminum powder and nickel powder; conductive whiskers such aszinc oxide and potassium titanate; conductive metal oxides such astitanium oxide; and polyphenylene derivatives.

The binder is a component which enhances binding of an active materialto a conductive material and current collector. The binder is commonlyadded in an amount of 1 to 40% by weight, based on the total weight ofthe compound including the anode active material. Examples of the binderinclude polyfluorovinylidene, polyvinyl alcohol, carboxymethylcellulose(CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrollidone, tetrafluoroethylene, polyethylene, polypropylene, ethylenepropylene diene terpolymer (EPDM), sulfonated EPDM, styrene butadienerubber, fluoro rubbers and various copolymers.

The filler is a component optionally used to inhibit expansion of thecathode. There is no particular limit to the filler, so long as it doesnot cause adverse chemical changes in the fabricated battery and is afibrous material. As examples of the filler, there may be used olefinpolymers such as polyethylene and polypropylene; and fibrous materialssuch as glass fibers and carbon fibers.

The anode is prepared by applying an anode active material to an anodecurrent collector, followed by drying. The anode active material mayfurther comprise the afore-mentioned ingredients.

The anode current collector is generally fabricated to have a thicknessof 3 to 500 μm. There is no particular limit to the anode currentcollector, so long as it has suitable conductivity without causingadverse chemical changes in the fabricated battery. As examples of theanode current collector, mention may be made of copper, stainless steel,aluminum, nickel, titanium, sintered carbon, and copper or stainlesssteel surface-treated with carbon, nickel, titanium or silver, andaluminum-cadmium alloys. Similar to the cathode current collector, ifnecessary, these current collectors may also be processed to form fineirregularities on the surface thereof so as to enhance adhesion to theanode active materials. In addition, the current collectors may be usedin various forms including films, sheets, foils, nets, porousstructures, foams and non-woven fabrics.

In addition, examples of anode active materials that can be used in thepresent invention include carbons such as hard carbons and graphitecarbons; metal composite oxides such as Li_(y)Fe₂O₃ (0≦y≦1),Li_(y)WO₂(0≦y≦1), Sn_(x)Me_(1−x)Me′_(y)O_(z) (Me: Mn, Fe, Pb, Ge;Me′:Al, B, P, Si, Group I, II and III elements of the Periodic Table,halogens; 0<x≦1; 1≦y≦3; 1≦z≦8); lithium metals; lithium alloys;silicon-based alloys; tin-based alloys; metal oxides such as SnO, SnO₂,PbO, PbO₂, Pb₂O₃, Pb₃O₄, Sb₂O₃, Sb₂O₄, Sb₂O₅, GeO, GeO₂, Bi₂O₃, Bi₂O₄,Bi₂O₅ and the like; conductive polymers such as polyacetylene; andLi—Co—Ni materials.

The separator is interposed between the cathode and anode. As theseparator, an insulating thin film having high ion permeability andmechanical strength is used. The separator typically has a pore diameterof 0.01 to 10 μm and a thickness of 5 to 300 μm. As the separator,sheets or non-woven fabrics made of an olefin polymer such aspolypropylene and/or glass fibers or polyethylene, which have chemicalresistance and hydrophobicity, are used. When a solid electrolyte suchas a polymer is employed as the electrolyte, the solid electrolyte mayalso serve as both the separator and electrolyte.

The lithium salt-containing, non-aqueous electrolyte is composed of anon-aqueous electrolyte and a lithium salt. As the non-aqueouselectrolyte, a non-aqueous electrolytic solution, solid electrolyte andinorganic solid electrolyte may be utilized.

As the non-aqueous electrolytic solution that can be used in the presentinvention, for example, mention may be made of aprotic organic solventssuch as N-methyl-2-pyrollidinone, propylene carbonate, ethylenecarbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate,gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydroxy Franc, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid triester, trimethoxy methane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate and ethylpropionate.

Examples of the organic solid electrolyte include polyethylenederivatives, polyethylene oxide derivatives, polypropylene oxidederivatives, phosphoric acid ester polymers, poly agitation lysine,polyester sulfide, polyvinyl alcohols, polyvinylidene fluoride, andpolymers containing ionic dissociation groups.

Examples of the inorganic solid electrolyte include nitrides, halidesand sulfates of lithium such as Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH,LiSiO₄, LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH andLi₃PO₄—Li₂S—SiS₂.

The lithium salt is a material that is readily soluble in theabove-mentioned non-aqueous electrolyte and may include, for example,LiCl, LiBr, LiI, LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂,LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroboranelithium, lower aliphatic carboxylic acid lithium, lithium tetraphenylborate and imide.

Additionally, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride or the like may be added to the non-aqueouselectrolyte. If necessary, in order to impart incombustibility, thenon-aqueous electrolyte may further include halogen-containing solventssuch as carbon tetrachloride and ethylene trifluoride. Further, in orderto improve high-temperature storage characteristics, the non-aqueouselectrolyte may additionally include carbon dioxide gas.

Advantageous Effects

As apparent from the fore-going, the present invention provides acathode active material which is based on lithium transition metal oxidehaving a layered crystal structure, wherein an average oxidation numberof transition metals except lithium is higher than +3, and Ni³⁺ amongnickel corresponding to the manganese content is present in a relativelyhigher amount, thus realizing a uniform and stable crystal structure,and exhibiting superior overall electrochemical properties includingbattery capacity and considerably superior high-rate charge/dischargecharacteristics.

BEST MODE

Now, the present invention will be described in more detail withreference to the following Examples. These examples are provided onlyfor illustrating the present invention and should not be construed aslimiting the scope and spirit of the present invention.

EXAMPLE 1

Transition metal salts were dissolved in distilled water such that amolar ratio of nickel salt and manganese salt (Ni/Mn) was adjusted to1.12 and a molar ratio of a cobalt salt to the all transition metalsalts was adjusted to 9 mol %. Then, a transition metal composite wasobtained, while elevating the basicity of the aqueous transition metalsolution. The solvent was removed from the transition metal compositevia vacuum filtration and dried in an oven at 110□ for 18 hours toremove remaining solvent. The resulting composite had a structure ofM(OH_(1−x))₂ (in which M represents all transition metals and x has avalue of about 0.55 in accordance with thermogravimetric analysis). Thelithium salt was mixed such that the molar ratio of Ni²⁺/Mn⁴⁺ wasadjusted to 0.88, heated in an electric furnace at an elevating rate of300° C./hour up to about 960° C. and sintered at this temperature for 10hours to obtain a lithium transition metal composite oxide.

EXAMPLE 2

Lithium transition metal composite oxide was obtained in the same manneras in Example 1 except that the lithium salt was mixed such that themolar ratio of Ni²⁺/Mn⁴⁺ was 0.75.

COMPARATIVE EXAMPLE 1

Lithium transition metal composite oxide was obtained in the same manneras in Example 1 except that the lithium salt was mixed such that themolar ratio of Ni²⁺/Mn⁴⁺ was 1.

EXAMPLE 3

Lithium transition metal composite oxide was obtained in the same manneras in Example 1 except that the lithium salt was mixed such that themolar ratio of nickel salt to manganese salt (Ni/Mn) was 1.06 and themolar ratio of Ni²⁺/Mn⁴⁺ was 0.88.

EXAMPLE 4

Lithium transition metal composite oxide was obtained in the same manneras in Example 3 except that the lithium salt was mixed such that themolar ratio of Ni²⁺/Mn⁴⁺ was 0.76.

COMPARATIVE EXAMPLE 2

Lithium transition metal composite oxide was obtained in the same manneras in Example 3 except that the lithium salt was mixed such that themolar ratio of Ni²⁺/Mn⁴⁺ was 1.

EXPERIMENTAL EXAMPLE 1

The cathode active materials prepared in Examples 1 to 4 and ComparativeExamples 1 and 2 were thoroughly mixed with NMP such that a weight ratio(wt %) of an active material:a conductive material:a binder was95:2.5:2.5. The mixture thus obtained was applied to a 20 μm Al foil anddried at 130□ to obtain a cathode. The cathode thus obtained wasroll-pressed to have a porosity of about 25% and punched in the form ofa coin with an area of 1.487 cm². Li-metal was used as the counterelectrode of the punched cathode and a coin-shaped battery was obtainedusing an electrolyte solution of 1M LiPF6 in a solvent ofEC:DMC:DEC=1:2:1.

The first cycle discharge capacity and efficiency of the battery wereobtained through 0.1 C charge/0.1 C discharge, a ratio of 1 C or 2 Cdischarge capacity to 0.1 C charge was calculated as a percentage, ratecapability was measured and the results thus obtained are shown in Table1 below:

EXPERIMENTAL EXAMPLE 2

The cathode active materials prepared in Examples 1 to 4 and ComparativeExamples 1 and 2 were subjected to XRD, the structure thereof wasanalyzed through Retveld-refinement, a ratio of Ni (Ni²⁺) present in theLi site was obtained and the results thus obtained are shown in Table 1below.

TABLE 1 Average Ratio of 1^(st) cycle 1C 2C oxidation Ni (Ni²⁺) dis-l^(st) cycle discharge/ discharge/ number intercal- charge effi- 0.1C0.1C of ated capacity ciency discharge discharge transition into Li(mAh/g) (%) (%) (%) metals site (%) Ex. 1 153.5 89.6 90.6 86.1 3.05 3.52Ex. 2 148.3 90.5 91.8 87.4 3.10 2.83 Ex. 3 151.1 89.1 90.4 85.8 3.053.44 Ex. 4 145.6 91.2 91.3 87.2 3.10 2.72 Comp. 138.3 84.5 76.2 68.73.00 5.84 Ex. 1 Comp. 136.8 82.6 73.6 64.2 3.00 6.13 Ex. 2

As can be seen from Table 1 above, all lithium secondary batteries basedon the cathode active materials according to the present inventionexhibited a 1^(st) discharge capacity of 145. 6 mAh/g and a 1^(st)charge efficiency of at least 89.1%. In addition, all lithium secondarybatteries based on the cathode active materials according to the presentinvention exhibited a 2 C discharge capacity/0.1 C discharge capacity ofat least 85.8%.

The ratio of Ni (Ni²⁺) intercalated into Li site (%) in Table 1 meansthe ratio of Ni (Ni²⁺) which shares in the total Li site, i.e., sharingratio. The cathode active materials according to the present inventionhad an average oxidation number of the transition metals higher than +3and a ratio of Ni (Ni²⁺) intercalated into Li site (%) of 3.52 or less.

These results demonstrate that the cathode active material of thepresent invention increases the oxidation number of the transition metallayer, thereby increasing the size difference between the lithium ionand the transition metal ion and thus improving structural stability andelectrochemical properties.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

The invention claimed is:
 1. A lithium transition metal oxide having anα-NaFeO₂ layered crystal structure, as a cathode active material forlithium secondary battery, wherein the transition metal includes a blendof Ni and Mn, an average oxidation number of the transition metalsexcept lithium is +3 or higher, and the lithium transition metal oxidesatisfies Equations 1 and 2 below:1.0<m(Ni)/m(Mn)  (1)m(Ni²⁺)/m(Mn⁴⁺)<1  (2) wherein m(Ni)/m(Mn) represents a molar ratio ofnickel to manganese and m (Ni²⁺)/m (Mn⁴⁺) represents a molar ratio ofNi²⁺ to Mn⁴⁺.
 2. The cathode active material according to claim 1,wherein the average oxidation number of transition metals is higher than3.0 and not higher than 3.5.
 3. The cathode active material according toclaim 2, wherein the average oxidation number of the transition metalsexcept lithium is 3.01 to 3.3.
 4. The cathode active material accordingto claim 2, wherein the average oxidation number of the transitionmetals except lithium is 3.1 to 3.3.
 5. The cathode active materialaccording to claim 1, wherein the nickel is composed of nickel (a)present in an excessive amount, as compared to the manganese content andnickel (b) present in an amount corresponding to the manganese content.6. The cathode active material according to claim 5, wherein the nickel(b) present in an amount corresponding to the manganese content containsNi²⁺ and Ni³⁺.
 7. The cathode active material according to claim 5,wherein the nickel (a) is present in an excessive amount as compared tothe manganese content is Ni³⁺.
 8. The cathode active material accordingto claim 1, wherein the content of nickel intercalated into the lithiumsite is lower than 5 mol %.
 9. A cathode comprising the cathode activematerial according to claim
 1. 10. A lithium secondary batterycomprising the cathode according to claim 9.